Secondary Storage: Hard Disk Drive (HDD), Optical Discs & Solid-State (SSD)

An HDD stores binary data magnetically on circular spinning platters coated with ferromagnetic thin-film material: • Spindle Motor: Spins stacked magnetic platters at constant angular velocity (5400, 7200, 10000, or 15000 RPM). • Platters & Surfaces: Each platter has two recording surfaces (Top & Bottom) serviced by dedicated Read/Write electromagnetic heads. • Tracks: Concentric circular recording rings on platter surfaces (numbered from Track 0 at outer edge). • Sectors: The smallest addressable unit of disk storage (traditional: 512 Bytes; modern Advanced Format: 4096 Bytes / 4 KB). • Cylinders: The set of all corresponding tracks across all platter surfaces at the same radial arm position.

Total Average Disk Access Time is the cumulative sum of 3 mechanical and electrical delays: 1. Seek Time (T_seek): The mechanical time required for the actuator arm to move and position the Read/Write head over the target track cylinder (Typically 4 – 10 ms). 2. Rotational Latency (T_latency): The average time spent waiting for the spinning platter to bring the target sector under the stationary head. On average, the platter must rotate half a revolution (1/2 turn): • Average Rotational Latency (ms) = 1/2 * (60 / RPM) * 1000 ms. • For 7200 RPM: T_latency = 0.5 * (60,000 / 7200) = 4.17 ms. • For 5400 RPM: T_latency = 0.5 * (60,000 / 5400) = 5.56 ms. 3. Data Transfer Time (T_transfer): The time required to read/stream data bits into host memory: • Transfer Time = (Data Block Size in MB) / (Transfer Rate in MB/s). • Total Disk Access Time = Seek Time + Rotational Latency + Data Transfer Time.

Optical discs store digital data as microscopic physical indentations (Pits) and flat reflective areas (Lands) along a single continuous spiral track, read via laser reflection: • Compact Disc (CD-ROM): 700 MB capacity. Uses Infrared Laser (780 nm wavelength). Track pitch: 1.6 µm. • Digital Versatile Disc (DVD): 4.7 GB (Single Layer) / 8.5 GB (Dual Layer). Uses Red Laser (650 nm wavelength). Track pitch: 0.74 µm. • Blu-Ray Disc (BD): 25 GB (Single Layer) / 50 GB (Dual Layer). Uses Blue-Violet Laser (405 nm wavelength). Shorter wavelength creates microscopic focal spots allowing 5x denser data tracks!

• SSD Architecture: Built from non-volatile NAND Flash memory controllers and DRAM cache buffers. • Flash Cell Types: SLC (Single-Level Cell: 1 bit/cell - fastest, durable), MLC (2 bits/cell), TLC (3 bits/cell - consumer standard), QLC (4 bits/cell - budget high capacity). • SSD Advantages: Zero mechanical seek latency (< 0.1 ms access), 500 – 7000 MB/s transfer speeds (SATA III / NVMe PCIe Gen4/5), silent operation, shock and drop resistant, low power consumption.